1. Field of the Invention
The present invention relates to a multifunctional optical film, and a surface light source device and a liquid crystal display employing the optical film. More particularly, the present invention relates to a multifunctional optical film having light transmission, light diffusion, heat resistance, UV-shielding properties, etc., due to the use of optical silicone resin(s), and showing improved production efficiency based on physical properties (e.g., release property, smoothness) of the silicone resin(s), and a surface light source device and a liquid crystal display employing the optical film.
2. Description of the Related Art
Recently, plasma display panels (PDPs), field emission displays (FEDs), thin film transistor-liquid crystal displays (TFT-LCDs) etc. have been developed as flat panel displays used in notebook computers, televisions, mobile phones, etc. requiring thinness, compactness, and low power consumption. Among them, LCDs with good color reproducibility and thin thickness have been the most actively studied.
Unlike PDPs and FEDs which are self-emissive displays, LCDs are not self-emissive, and thus, use a backlight which is an auxiliary light source illuminating a rear side of an LCD panel to achieve displays. In order to illuminate an LCD panel, and further, to uniformly illuminate the entire of the LCD panel, a backlight has a surface light source structure called as an edge-type as illustrated in FIG. 1 or as a direct-type as illustrated in FIG. 2. FIG. 1 schematically illustrates a conventional LCD including an edge-type light source. Referring to FIG. 1, an LCD includes a light source 11, a light guide plate 12 guiding light emitted from the light source 11, a reflective plate 13 disposed on a lower surface of the light guide plate 12, a diffusing sheet 14 disposed on an upper surface of the light guide plate 12, a prism sheet 15 horizontally or vertically disposed on an upper surface of the diffusing sheet 14, and a protective sheet 16 disposed on an upper surface of the prism sheet 15. The light source 11 is covered with a light source cover 11a. FIG. 2 schematically illustrates a conventional LCD including a direct-type light source. Referring to FIG. 2, an LCD includes a plurality of light sources 21 disposed to be spaced apart from each other by a predetermined distance, a reflective plate 23 disposed below the light sources 21, a protective plate (not shown) disposed on a lower surface of the reflective plate 23, a diffusing sheet 24 disposed on an upper surface of the light sources 21, a prism sheet 25 disposed on an upper surface of the diffusing sheet 24, and a protective sheet 26.
In particular, as a recent trend for TFT-LCDs is toward increasing the sizes of the TFT-LCDs, there arise problems such as a reduction in contrast which is one of the most major disadvantages, accelerated aging of a film by over-exposure to UV, or a reduction in brightness due to a curl phenomenon.
As examples of light-diffusing films used hitherto, there are (1) a diffusing sheet obtained by forming a transparent thermoplastic resin in the form of a sheet and physically forming a concavo-convex pattern on a surface of the sheet (see Japanese Patent Laid-Open Publication No. Hei. 4-275501), (2) a light-diffusing film obtained by coating a light-diffusing layer formed of a transparent resin containing fine particles on a transparent substrate film formed of a polyester resin (see Japanese Patent Laid-Open Publication No. Hei. 6-59108), (3) a light-diffusing sheet obtained by melt-blending of beads with a transparent resin and extruding the molten blend (see Japanese Patent Laid-Open Publication No. Hei. 6-123802), and (4) a light-diffusing sheet (a light-diffusing film) having an islands-in-sea structure obtained by melt-mixing of two or more transparent thermoplastic resins (see Japanese Patent Laid-Open Publication No. Hei. 9-311205).
The light-diffusing films of (1) and (2) are so-called surface light-diffusing films showing a light-diffusing effect by means of a concavo-convex surface pattern or a coated light-diffusing layer. On the other hand, the light-diffusing films of (3) and (4) are light-diffusing films containing light-diffusing components at least inside the substrates.
Of these, the light-diffusing film of (2) obtained by coating a light-diffusing layer on a transparent substrate film has been currently widely used. Generally, a biaxially-drawn polyethyleneterephthalate (PET) film is mainly used as a transparent substrate film. The biaxially-drawn PET film is well known to have good mechanical strength, heat resistance, transparency, and smoothness. When such a PET film is used as a substrate film, an optical film including the substrate film also has the properties of the PET film.
In addition, high performance, high efficiency, thinness, lightness, etc. have been required in the field of constitutional members of LCDs. In order to satisfy the requirements, for example, multi-functionality by surface processing, stacking of films, etc. have been considered. However, the light-diffusing films of (1) and (2) above have considerable surface irregularities, are difficult to be stacked one onto another, together with other films, and cannot be actually surface-processed. With respect to the light-diffusing films of (3) and (4), a diffusion effect is essentially obtained by diffusing components contained in the films. The light-diffusing films of (3) and (4) have a smoother surface than those of (1) and (2), but the smoothness of the light-diffusing films of (3) and (4) cannot be said to be sufficient due to surface irregularities by beads or a thermoplastic resin constituting an islands-in-sea structure in the vicinity of a surface layer. Moreover, when crosslinkable organic microparticles or inorganic microparticles, e.g., beads are melt-blended, like the light-diffusing film of (3), there may occur clogging of filters which are inserted into an extruder in order to remove impurities according to the shape or size of the microparticles, and fluidity of a resin composition may worsen at the time of melting, according to the addition amount of the microparticles, such that film formation is impossible. In particular, films containing therein considerable amounts of diffusing components (beads, etc.), like the light-diffusing films of (3) and (4), include no support, and thus, there is a tendency for strength, in particular flexural strength, to be low. For example, creases may be readily introduced, thereby causing a whitening phenomenon, or creasing or splitting may occur at the edges ate the time of cutting. Moreover, when installed in backlight units, etc., the light-diffusing films of (3) and (4) may be degraded due to temperature elevation of constitutional member(s) by long-time illumination of backlights. As such, when the light-diffusing films of (3) and (4) are installed in backlight units and used for a long time, film distortion may occur, thereby resulting in brightness variation of backlights.
A light-focusing film includes a substrate and a resin film that are different in refractive index in order to enhance directionality of light, and focuses incident light by fine patterns at a surface of the resin film. The fine patterns significantly affect a viewing angle, haziness, light-focusing efficiency, etc. according to the shape, size, arrangement, etc. of the fine patterns, and thus, the shape, size, arrangement, etc. of the fine patterns are important factors that must be considered next to a refractive index difference between a substrate and a photo-curable resin. The absolute value of a refractive index difference between a substrate and a photo-curable resin may be 0.001 or more, more preferably 0.005 to 1, and most preferably 0.1 or more. Light directionality, scattering, diffusion, and focusing properties are also significantly affected by a film material and thickness of a substrate, a volumetric fraction of fine patterns and a film thickness of a photo-curable resin. In particular, front brightness can be maximized by reducing a light absorption (light loss) value of a coating material. That is, brightness, haziness, and a viewing angle can be precisely controlled by appropriately selecting the above-described conditions.
With respect to a fine concavo-convex pattern (e.g., spherical pattern, lens pattern, prism pattern) constituting a light-diffusing or light-focusing film, a prism pattern is preferred due to easy and precise adjustment of brightness, haziness, and a viewing angle. Here, the term “prism pattern” refers to a prism pattern with a triangular sectional profile (hereinafter, referred to as simply “triangular prism pattern”). Although a prism pattern does not necessarily have a complete triangular sectional profile, a non-anisotropic prism pattern is preferred in order to uniformly focus light.
In order to achieve an enhancement in transmittance (front brightness) which is the most important factor among the performance factors of a film to enhance brightness, it is important to consider the wavelength of light and the shape and pattern pitch of a fine concavo-convex pattern. Enhancement in transmittance can be achieved by adjusting the pattern pitch of a concavo-convex pattern to 0.1 to 5 μm. If the pattern pitch of a fine concavo-convex pattern is less than 0.1 μm, the color of transmitted light may be visualized. Thus, by adjusting the pattern pitch of a fine concavo-convex pattern to the above range, unwanted coloration of transmitted light can also be prevented. In addition, a fine concavo-convex surface pattern of a resin film protects a constitutional member contacting with the resin film, and at the same time, has appropriate diffusion property, thereby providing hiding property which hides dots of a light guide plate. Such a resin film is disposed on at least one of upper and lower surfaces of a transparent substrate.
In conventional LCDs, constitutional members (e.g., a diffusing sheet, a prism sheet, a brightness enhancement sheet, a polarization plate, a phase contrast plate, a liquid crystal material, a color filter) of a surface light source unit are degraded by UV light leaked from light sources during long-time operation of the LCDs. In order to solve this problem, a method of protecting liquid crystal cells using a polarization plate protective film containing a UV absorber has been proposed (Japanese Patent Laid-Open Publication No. Hei. 11-246704).
In order to prevent UV leakage, placement of a UV-absorbing film as close as possible to fluorescent light in a backlight unit or addition of a UV absorber to a light guide plate can be considered. However, for the former method, it is necessary to use a film with good heat resistance, and for the latter method, a whole color change may occur due to partial absorption of visible light by the UV absorber. Recently, a method of converting a trace amount of UV light from a fluorescent tube to visible light using a UV absorber (e.g., magnesium oxide or titanium oxide) has been proposed to prevent UV leakage. According to this method, however, strong UV light is leaked from a light guide plate, and thus, when used for a long time, members constituting a surface light source unit appear yellowish.
As described above, various additives may be added to light-diffusing films in an amount such that a light-diffusing effect is not damaged. For example, the additives may be pigments, dyes, optical brightening agents, antioxidants, heat stabilizers, light stabilizers, weathering agents, antistatic agents, release agents, compatibilizing agents, etc. In particular, a method of offering diffusion property to a light-diffusing film in the presence of additives inevitably involves the use of a diffusion material in a resin. This changes a concavo-convex surface pattern, thereby making it difficult to precisely adjust brightness, haziness, and a viewing angle. In particular, a concavo-convex surface pattern is deformed and/or discolored by UV light emitted from an optical lamp, thereby adversely affecting optical characteristics and anti-sticking property.
When such a light-diffusing film is installed in a backlight unit, a side or a rear surface of the light-diffusing film is directly contacted to a light source (e.g., a lamp) due to the structural feature of an LCD. Thus, the light-diffusing film must be stabilized against heat generated when the light source is kept at an “ON” state. A plastic support film is thermally shrunk due to repeated generation and annihilation of heat by a light source. Such thermal shrinkage deforms an optical film fixedly placed in a predetermined frame such that a center portion of the optical film expands. As a result, uniform light transmission is not achieved, thereby making unwanted predetermined patterns on an LCD screen. Moreover, an adhesion power of a plastic support with a light-focusing layer coated on the plastic support is decreased, and thus, the light-focusing layer is separated from the plastic support with time, thereby adversely affecting uniform light-focusing and light-transmission properties, thereby resulting in a reduction in brightness of an LCD screen.